Spatial null creation using massive MIMO (M-MIMO)

ABSTRACT

In a base station having a Massive Multiple Input Multiple Output (M-MIMO) antenna array, the availability of the M-MIMO antenna array is exploited to manage the interference caused by the base station to neighboring cells. In one embodiment, the large number of antenna elements of the M-MIMO antenna array are used to create precise transmit and/or receive spatial nulls at specific User Equipments (UEs) being served by a neighboring cell and/or in select areas of the neighboring cell. Depending on whether the spatial null is partial or full, transmissions by the base station may have reduced or even zero receive power within the neighboring cell.

This application is a continuation of U.S. application Ser. No.14/137,222, filed on Dec. 20, 2013, now U.S. Pat. No. 9,300,501, whichclaims the benefit of U.S. Provisional Applications No. 61/811,563,filed on Apr. 12, 2013, and 61/813,337, filed on Apr. 18, 2013, all ofwhich are incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure relates generally to spatial null creation usinga Massive Multiple Input Multiple Output (MIMO) (M-MIMO) antenna array.

Background Art

In a Massive Multiple Input Multiple Output (M-MIMO) communicationsystem, a transmitter, such as a base station, is equipped with a verylarge number of transmit antennas (e.g., 32, 64, or 100) that can beused simultaneously for transmission to one or more receivers, such as auser equipment (UE).

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present disclosure and, togetherwith the description, further serve to explain the principles of thedisclosure and to enable a person skilled in the pertinent art to makeand use the disclosure.

FIG. 1 illustrates an example environment in which embodiments can beimplemented or practiced.

FIG. 2 illustrates another example environment in which embodiments canbe implemented or practiced.

FIG. 3 illustrates an example base station according to an embodiment.

FIGS. 4-8 illustrate example process according to embodiments.

The present disclosure will be described with reference to theaccompanying drawings. Generally, the drawing in which an element firstappears is typically indicated by the leftmost digit(s) in thecorresponding reference number.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of this discussion, the term “module” shall be understoodto include at least one of software, firmware, and hardware (such as oneor more circuits, microchips, processors, or devices, or any combinationthereof), and any combination thereof. In addition, it will beunderstood that each module can include one, or more than one, componentwithin an actual device, and each component that forms a part of thedescribed module can function either cooperatively or independently ofany other component forming a part of the module. Conversely, multiplemodules described herein can represent a single component within anactual device. Further, components within a module can be in a singledevice or distributed among multiple devices in a wired or wirelessmanner.

FIG. 1 illustrates an example environment 100 in which embodiments canbe implemented or practiced. Example environment 100 is provided for thepurpose of illustration only and is not limiting of embodiments. Asshown in FIG. 1, example environment 100 includes a first base station102 and a second base station 104. In an embodiment, first base station102 includes a Massive Multiple Input Multiple Output (M-MIMO) antennaarray 110 comprised of a plurality of antenna elements. M-MIMO antennaarray 110 can be a uniform array (1D, 2D, or 3D), with uniform spacingbetween antenna elements, or a non-uniform array. The number of antennaelements of M-MIMO antenna array 110 is significantly larger than thenumber of antenna elements used in existing base station implementations(which can be up to 8 antenna elements). For example, M-MIMO antennaarray 110 can have 16, 32, 64 or more antenna elements.

In an embodiment, first base station 102 is a high power base station(e.g., macrocell) that provides cellular service within a coverage area106. For example, first base station 102 may serve a plurality of userequipments (UEs), such as UEs 112 a, 112 b, and 112 c, within coveragearea 106. Second base station 104 is a low power base station (e.g.,macrocell or femtocell) that provides cellular service within a coveragearea 108. For example, second base station 104 may serve a plurality ofUEs, such as UEs 114 a and 114 b, within coverage area 108. As would beunderstood by a person of skill in the art based on the teachingsherein, embodiments are not limited by this example. For example, firstbase station 102 and second base station 104 may both be high power orlow power base stations in other embodiments.

In an embodiment, as shown in example environment 100, coverage area 106of first base station 102 fully encompasses the geographic area definedby coverage area 108 of second base station 104. In another embodiment,as shown in example environment 200 of FIG. 2, coverage area 106 offirst base station 102 and coverage area 108 of second base station 104overlap with each other in an intersection area 202. Because first basestation 102 and second base station 104 can use the same time andfrequency resources to serve their respective UEs, first base station102 and second base station 104 may interfere with each other withincoverage area 108 or intersection area 202. Specifically, in exampleenvironment 100, UEs 114 a and 114 b as well as second base station 104can experience high receiver interference due to first base station 102.This is especially true when first base station 102 is a high power basestation and second base station 104 is a low power base station. Inexample environment 200, UE 114 a located in intersection area 202 mayexperience high receiver interference due to first base station 102.Similarly, first base station 102 may experience high interference dueto second base station 104, UE 114 a, and/or UE 114 b in exampleenvironment 100 and due to UE 114 a in example environment 200.

Existing solutions to manage interference in example environments 100and 200 include Time Division Multiple Access (TDMA) and FrequencyDivision Multiple Access (FDMA) solutions. TDMA solutions require one orboth of base stations 102 and 104 to stop transmitting at set periods oftime to alleviate interference at the other base station. FDMA solutionsdivide available resources in the frequency domain so that resourcesused by first base station 102 are orthogonal to resources used bysecond base station 104. Alternatively, first base station 102 and/orsecond base station 104 may use lower power on specified subcarriers toreduce interference on the specified subcarriers. Because existingsolutions effectively divide available time and frequency resourcesbetween the interfering base stations, they come at the expense ofreduced system capacity, which makes them unsuitable for future highthroughput cellular networks.

Embodiments, as further described below, exploit the availability ofM-MIMO antenna array 110 at first base station 102 to manage theinterference in example environments 100 and 200, for example, in amanner such that full simultaneous operation by the multiple basestations (generally without any Time Division Multiple Access (TDMA),Frequency Division Multiple Access (FDMA), and/or power control schemes;though embodiments do not preclude the additional use of such schemes)is possible. In one embodiment, the large number of antenna elements ofM-MIMO antenna array 110 are used by first base station 102 to createprecise transmit and/or receive spatial nulls in select directionsand/or in select areas within coverage area 106. In one embodiment,first base station 102 can use M-MIMO antenna array 110 to create aspatial null within or over entire coverage area 108 of second basestation 104. Depending on whether the spatial null is partial or full,transmissions by first base station 102 may have reduced or even zeroreceive power within coverage area 108, causing minimal or nointerference.

Example embodiments are now presented. For the purpose of illustrationonly, the example embodiments are described with reference to exampleenvironments 100 and 200 discussed above and with reference to anexample base station 300 illustrated in FIG. 3 described below. As wouldbe understood by a person of skill in the art based on the teachingsherein, embodiments are not limited by these example environments orexample base station implementation.

FIG. 3 illustrates an example base station 300 according to anembodiment. Example base station 300 is provided for the purpose ofillustration only and is not limiting of embodiments. Example basestation 300 may be an embodiment of first base station 102 in exampleenvironments 100 and 200 discussed above. As shown in FIG. 3, examplebase station 300 includes, without limitation, a processor 302; aswitching module 304; an antenna array controller 306, comprising aplurality of antenna controllers 306.0, 306.1, . . . , 306.n; a M-MIMOantenna array 312, comprising a plurality of antenna elements 312.0,312.1, . . . , 312.n; and a memory 314.

In an embodiment, processor 302 includes a baseband processor whichgenerates one or more (e.g., N) data streams (not shown in FIG. 3) fortransmission by base station 300 over the same time and frequencyresources. The data streams each typically comprises a sequence ofmodulated data symbols. The data streams can be different from eachother. Alternatively, some of the data streams can be duplicate.

The data streams are generally intended for one or more UEs (e.g., KUEs) served by base station 300. For example, referring to FIG. 1, theone or more UEs may be one or more of UEs 112 a-c served by first basestation 102. A UE served by base station 300 may be the intendedrecipient of one or more or none of the data streams transmitted by basestation 300. For example, referring to FIG. 1, UE 112 a may not be anintended recipient of any of the data streams, and as such base station300 can ensure that no or minimal power due to the transmission of theplurality of data streams is received by UE 112 a. UE 112 b may be theintended recipient of a single data stream of the data streams andaccordingly is said to have a rank equal to 1. UE 112 c may be theintended recipient of two data streams of the data streams andaccordingly is said to have a rank equal to 2.

In an embodiment, before forwarding the data streams to M-MIMO antennaarray 312 for transmission, processor 302 can pre-code the data streamsby applying a transmit precoder matrix to the data streams. The transmitprecoder matrix reduces to a transmit precoder vector in the case of asingle data stream being transmitted; in the following the term transmitprecoder matrix is used, but it would apparent to a person of skill inthe art that embodiments also include the use of a transmit precodervector. Typically, the transmit precoder matrix is selected based onpartial or full knowledge of the downlink channel over which the datastreams will be transmitted. As described above, the downlink channelcan be a MIMO channel, with multiple downlink channel paths from everytransmit antenna to every receive antenna.

In an embodiment, pre-coding the data streams using the transmitprecoder matrix generates pre-coded data streams. In an embodiment, if amulti-subcarrier system is used (e.g., Orthogonal Frequency DivisionMultiplexing), the pre-coding can be performed on a sub-carrier bysub-carrier basis. Depending on the actual values of the transmitprecoder matrix, the pre-coded data streams can each correspond to anamplitude and/or phase adjusted version of a single respective datastream (where the transmit precoder matrix is a diagonal matrix), or oneor more of the pre-coded data streams can be a weighted combination oftwo or more of the data streams. In the former case, an antenna elementof M-MIMO antenna array 312 (which is used for transmission) wouldtransmit a signal containing a single data stream only. In the lattercase, the antenna element would transmit a signal containing a weightedcombination of two or more data streams.

As further described below, in embodiments, processor 302 can select thetransmit precoder matrix to ensure no or minimal interference within acoverage area of a neighboring base station and/or at select UEs withinthe coverage area of the neighboring base station. In other words, thetransmit precoder matrix can be selected to produce a desired transmitbeam pattern of M-MIMO antenna array 312. This functionality is enabledby the large number of transmitter degrees of freedom afforded by M-MIMOantenna array 312, which allows for the shaping of the transmit beampattern as desired. For example, in an embodiment, processor 302 canselect the transmit precoder matrix such that each data stream isreceived by its intended UE recipient without no or minimal interferencedue to the other data streams. In another embodiment, the transmitprecoder matrix can be further configured such that no or minimal powerdue to the transmission of the data streams is received within acoverage area of a neighboring base station and/or at select UEs withinthe coverage area of the neighboring base station. In a furtherembodiment, the transmit precoder matrix can be further configured tobeamform to one or more UEs served by base station 300.

Similarly, in the receive direction, processor 302 can apply a receivedecoding matrix to data streams received from M-MIMO antenna array 312,which is typically followed by standard receive processing modules suchas equalization and FEC decoding to generate separated data streams. Thereceive decoding matrix reduces to a receive decoding vector in the caseof a single data stream being received; in the following the termreceive decoding matrix is used, but it would apparent to a person ofskill in the art that embodiments also include the use of a receivedecoding vector. The received decoding matrix can be selected to producea desired receive beam pattern. In an embodiment, the receive decodingmatrix can be selected to produce a receive beam pattern with a spatialnull in the direction of a neighboring base station and/or one or moreUEs served by the neighboring base station. This has the effect ofnulling, at base station 300, transmissions from the neighboring basestation and/or the one or more UEs served by the neighboring basestation. In another embodiment, the receive decoding matrix can befurther configured to perform receive beamforming from one or more UEsserved by base station 300. Like the transmit precoder matrix, thereceive decoding matrix can be selected based on partial or fullknowledge of the uplink channel over which the data streams arereceived. The uplink channel can be a MIMO channel, with multiple uplinkchannel paths from every transmit antenna to every receive antenna.

Switching module 304 is coupled to processor 302. In an embodiment,switching module 304 is controllable by processor 302 by means of acontrol signal 316 in order to couple the data streams from processor302 to M-MIMO antenna array 312 or to couple signals received by M-MIMOantenna array 312 to processor 302. In an embodiment, processor 302determines a subset (which may include all) of the plurality of antennaelements 312.0, 312.1, . . . , 312.n of M-MIMO antenna array 312 totransmit the data streams or to receive signals transmitted to basestation 300. In an embodiment, processor 302 selects the subset of theplurality of antenna elements 312.0, 312.1, . . . , 312.n in accordancewith the desired transmit beam pattern or in accordance with the desiredreceive beam pattern. For example, this may include selecting the subsetof the plurality of antenna elements 312.0, 312.1, . . . , 312.n ashaving an appropriate geometry to produce the desired transmit beampattern given any transmit precoder matrix being applied to the datastreams or to produce the desired receive beam pattern given any receivedecoding matrix being applied to the received data streams.

Antenna array controller 306 is coupled between switching module 304 andM-MIMO antenna array 312. In an embodiment, antenna array controller 306includes a plurality of antenna controllers 306.0, 306.1, . . . , 306.nthat correspond respectively to antenna elements 312.0, 312.1, . . . ,312.n of M-MIMO antenna array 312. In an embodiment, each antennacontroller 306.0, 306.1., . . . , 306.n includes a respective phasecontroller 308 and a respective amplitude controller 310. Antenna arraycontroller 306 can be implemented using digital and/or analogcomponents.

In an embodiment, processor 302 controls antenna array controller 306 bymeans of a control signal 318. In another embodiment, processor 302controls antenna array controller 306 using control signal 318 toactivate one or more of antenna controllers 306.0, 306.1, . . . , 306.ndepending on which of antenna elements 312.0, 312.1, . . . , 312.n isbeing used for transmission or reception. In an embodiment, when anantenna element 312.0, 312.1, . . . , 312.n is used for transmission orreception, its corresponding antenna controller 306.0, 306.1, . . . ,306.n is active. A phase shift can be applied to a signal beingtransmitted or received by an antenna element 312.0, 312.1, . . . ,312.n using its respective phase controller 308.0, 308.1, . . . , 308.n.An amplitude amplification/attenuation can be applied to a signal beingtransmitted or received using an antenna element 312.0, 312.1, . . . ,312.n using its respective amplitude controller 310.0, 310.1, . . . ,310.n. In an embodiment, the phase shift and amplitudeamplification/attenuation are applied in the time domain to the signal.

In an embodiment, processor 302 determines, based on one or more of: thedesired transmit beam pattern, the downlink channel, the selectedtransmit precoder matrix, and the subset of antenna elements used fortransmission, a transmit weight vector for antenna array controller 306.In an embodiment, the transmit weight vector includes a complex elementfor each antenna controller 306.0, 306.1, . . . , 306.n, whichdetermines the respective phase shift and amplitudeamplification/attenuation to be applied by the antenna controller to thesignal being transmitted by its respective antenna element. In anotherembodiment, processor 302 determines, based on one or more of: thedesired receive beam pattern, the uplink channel, the selected receivedecoding matrix, and the subset of antenna elements used for reception,a receive weight vector for antenna array controller 306. In anembodiment, the receive weight vector includes a complex element foreach antenna controller 306.0, 306.1, . . . , 306.n, which determinesthe respective phase shift and amplitude amplification/attenuation to beapplied by the antenna controller to the signal received by itsrespective antenna element. Hence, as described above, antenna arraycontroller 306 provides an additional layer for shaping the transmitbeam pattern or the receive beam pattern of M-MIMO antenna array 312.

According to embodiments, any combination of a transmit precoder matrixand a transmit weight vector (in addition to an appropriate selection ofa subset of antenna elements) can be used to produce a desired transmitbeam pattern of M-MIMO antenna array 312. Similarly, any combination ofa receive decoding matrix and a receive weight vector (in addition to anappropriate selection of a subset of antenna elements) can be used toproduce a desired receive beam pattern of M-MIMO antenna array 312. Inother words, a desired transmit/receive beam pattern according toembodiments can be achieved using precoding means and/or using perantenna phase/amplitude weighing. In the following, further embodimentswill be described. For simplification, embodiments describe only the useof a transmit precoder matrix or a receive decoder matrix to produce adesired transmit beam pattern or a desired receive beam pattern.However, it should be understood based on the teachings herein thatembodiments encompass the use of any combination of transmit precodermatrix and transmit weight vector and any combination of receivedecoding matrix and receive weight vector to produce desired transmitand receive beam patterns.

As mentioned above, in an embodiment, processor 302 can select thetransmit precoder matrix such that a transmit beam pattern of M-MIMOantenna array 312 creates a spatial null within or over the entirecoverage area of a neighboring base station. For example, referring toFIG. 1, first base station 102 can use its M-MIMO antenna array 110 tocreate a spatial null within or over entire coverage area 108 of secondbase station 104. Depending on whether the spatial null is partial orfull, transmissions by first base station 102 may have reduced or evenzero receive power within coverage area 108, causing minimal or nointerference.

In an embodiment, the downlink channel from base station 300 to thecoverage area of the neighboring base station is characterized a priorito enable spatial null creation. For example, at set up time of theneighboring base station (which may be a femto cell), the coverage areaof the neighboring base station can be traversed and pilot signals canbe transmitted from different locations of the coverage area. The pilotsignals received by base station 300 can be processed to determine theuplink channel from the coverage area to base station 300. The downlinkchannel can be estimated from the uplink channel by reciprocity.Alternatively, base station 300 can transmit downlink pilots to devicesplaced in various locations of the coverage area and receive channelfeedback from those devices to determine the downlink channel. Inanother embodiment, the downlink channel from base station 300 to thecoverage area can be characterized at different times of the day toprovide further granularity of the downlink channel.

In an embodiment, processor 302 can select the transmit precoder matrixsuch that the spatial null is constantly created within or over thecoverage area of the neighboring base station. In another embodiment,processor 302 can vary the transmit precoder matrix to selectivelycreate spatial nulls within or over the entire coverage area of theneighboring base station as illustrated in example process 400 of FIG. 4described below. Example process 400 is provided for the purpose ofillustration only and is not limiting of embodiments. Example process400 is described below with reference to example base station 300 but isnot limited by this description as would be understood by a person ofskill in the art based on the teachings herein.

As shown in FIG. 4, example process 400 begins in step 402, whichincludes determining a historical activity profile of a neighboring basestation. The historical activity profiles describe the activity levelsof neighboring base stations (e.g., average number of UEs served) overtime (e.g., time of the year, time of the month, time of the week, timeof the day). In an embodiment, base station 300 can create thehistorical activity profiles of neighboring base stations by monitoringtransmissions within the coverage areas of the neighboring basestations. In another embodiment, each base station creates its ownhistorical activity profile and shares it with neighboring basestations. In an embodiment, historical activity profiles of neighboringbase stations of base station 300 are stored in memory 314, andprocessor 302 can retrieve a historical activity profile of aneighboring base station from memory 314.

Subsequently, in step 404, process 400 includes creating partial, full,or no spatial nulling in a coverage area of the neighboring base stationbased on the historical activity profile of the neighboring basestation. In an embodiment, step 404 is performed by processor 302, whichcontrols a transmit beam pattern of M-MIMO antenna array 312 to createpartial, full, or no spatial nulling in the coverage area of theneighboring base station based on the historical activity profile of theneighboring base station. For example, if the historical activityprofile of the neighboring base station indicates a high activity levelat the time, then a full spatial null can be applied over the entirecoverage area of the neighboring base station. Alternatively, if thehistorical activity level indicates a low activity level at the time,then a partial spatial null (where some receive power due base station300 can be present) or no spatial nulling may be used.

In another embodiment, if the historical activity profile indicates lowactivity within the coverage area of the neighboring base station, forexample, then spatial nulls may be dynamically created at select UEsserved by the neighboring base station or at the neighboring basestation itself. A UE may be selected based on one or more of: the levelof interference caused by base station 300 at the UE (e.g., spatialnulling may be used for most affected UEs), the Quality of Service (QoS)desired by the UE (e.g., spatial nulling may be used for a UE requiringa high data rate connection), UE capabilities (e.g., presence ofadvanced interference suppression/rejection), and the mobility of the UE(e.g., spatial nulling may be used less for high mobility UEs). As such,in an embodiment, processor 302 can be configured to identify a UEserved by a neighboring base station and to control M-MIMO antenna array312 to create a spatial null at the UE.

In the following, example processes according to embodiments fordynamically creating a spatial null at a UE served by a neighboring basestation of base station 300 are presented. These example processes areprovided for the purpose of illustration only and are not limiting ofembodiments. While the example processes illustrate various ways tocreate a spatial null at a UE served by a neighboring base station, theycan be equally used to create a spatial null at a neighboring basestation or at any other UE within coverage of base station 300 as wouldbe understood by a person of skill in the art based on the teachingsherein.

Generally, the example processes can be performed by processor 302and/or other components of base station 300 as would be apparent to aperson of skill in the art based on the teachings herein. Briefly, theexample processes can include processor 302 being configured to:determine an estimate of a channel from base station 300 to a UE servedby a neighboring base station; determine, based at least in part on theestimate, a transmit precoder matrix (and/or a transmit weight vector);apply the transmit precoder matrix (and/or the transmit weight vector)to one or more data streams to generate one or more pre-coded datastreams; determine a subset of the plurality of antenna elements 312.0,312.1, . . . , 312.n of M-MIMO antenna array 312 to transmit the one ormore pre-coded data streams; and control switching module 304 to couplethe pre-coded data streams to the subset of the plurality of antennaelements 312.0, 312.1, . . . , 312.n of M-MIMO antenna array 312. Asdescribed above, in an embodiment, processor 302 is further configuredto determine at least one of the transmit precoder matrix and the subsetof the plurality of antenna elements of M-MIMO antenna array 312 toproduce a transmit beam pattern using M-MIMO antenna array 312 having aspatial null in a direction of the selected UE. Having a spatial null inthe direction of the selected UE ensures that the selected UE receivesno or minimal power due to the transmission of the data streams by basestation 300. In another embodiment, rather than creating a full spatialnull at the selected UE, base station 300 determines at least one of thetransmit precoder matrix and the subset of the plurality of antennaelements of M-MIMO antenna array 312 such that interference from basestation 300 is spatially aligned at the selected UE. In an embodiment,interference due to a plurality of data streams transmitted by basestation 300 can be configured to arrive in a single spatial direction atthe UE. The UE can perform single-direction interference cancellation tocancel out all of the interference due to base station 300. Only oneadditional antenna (in addition to the n antennas the UE requires todecode the n streams intended for it) at the UE is necessary to performthis cancellation; in contrast, conventionally, the UE would need thenumber of additional antennas to be at least as large as the number oftransmitted data streams to cancel out interference because theinterference would arrive from several spatial directions, one for eachstream. In another embodiment, the interference from K streams can bemade to arrive in strictly more than 1 spatial direction but strictlyfewer than K spatial directions at the selected UE, so that alignmentstrictly reduces the requirement on the number of antennas needed forcancellation by the UE but not to the extent of single-directionalignment.

In an embodiment, processor 302 can be further configured to determinethe estimate of the channel from base station 300 to the UE based onuplink pilot transmissions transmitted by the UE to the neighboring basestation, as further illustrated in example process 500 of FIG. 5.Example process 500 is provided for the purpose of illustration only andis not limiting of embodiments. Example process 500 is described belowwith reference to example base station 300 but is not limited by thisdescription as would be understood by a person of skill in the art basedon the teachings herein.

As shown in FIG. 5, example process 500 begins in step 502, whichincludes receiving uplink pilot transmissions transmitted by a UE to aneighboring base station. In an embodiment, the UE transmits periodicaluplink pilots to the neighboring base station, which serves the UE. Basestation 300 can have knowledge of the transmitted uplink pilots and thetime and frequency resources used for the uplink pilot transmissions viaa backhaul link that connects base station 300 and the neighboring basestation. In an embodiment, processor 302 can control M-MIMO antennaarray 312 (including controlling antenna array controller 306 and/orselecting an appropriate receive decoding matrix) in order to receivethe uplink transmissions of the UE.

Subsequently, step 504 includes determining an estimate of a downlinkchannel from base station 300 to the UE based on the received uplinkpilot transmissions. In an embodiment, step 504 is performed byprocessor 302, which can be configured to determine an uplink channelfrom the UE to base station 300 based on the uplink pilot transmissionsand then derive the estimate of the downlink channel to the UE based onthe uplink channel.

Process 500 terminates in step 506, which includes determining based onthe estimate of the downlink channel to the UE a transmit precodermatrix to transmit a plurality of data streams, wherein the transmitprecoder matrix is configured to produce a transmit beam pattern usingM-MIMO antenna array 312 having a spatial null in the direction of theUE or interference alignment as described earlier. As described above,the data streams may be intended to one or more UEs served by basestation 300. Accordingly, in an embodiment, the transmit precoder matrixis selected based on both the downlink channel to the (unintended) UEand the channel to the one or more intended UE recipients of the datastreams.

For example, in an embodiment, processor 302 can determine aconcatenated downlink channel that includes as its elements thechannel(s) from base station 300 to each UE being served by thetransmission and the channel(s) from base station 300 to any UEs ofneighboring cells for which spatial null creation is desired. As wouldbe understood by a person of skill in the art, the size of each channelfrom base station 300 to a particular UE is a function of the number ofantenna elements of M-MIMO antenna array 312 used for the transmissionand the number of receive antenna elements at the UE. In an embodiment,all UEs use the same number of receive antenna elements. In anotherembodiment, UEs can have different numbers of receive antenna elements.To accommodate this variation, in an embodiment, processor 302 canassume that all UEs have the same number of receive antenna elements asthe UE with the maximum number of receive antenna elements in formingthe concatenated downlink channel, but, for a UE with less than themaximum number of receive antennas, set to zero the elements of thechannel to the UE which correspond to a receive antenna element thatdoes not exist at the UE.

In an embodiment, processor 302 is configured to determine the transmitprecoder matrix to transmit the plurality of data streams based on theconcatenated downlink channel. In an embodiment, processor 302determines or selects the transmit precoder matrix such that whenmultiplied by the concatenated downlink channel results in a diagonalmatrix. This effectively creates spatially orthogonal downlink pathsfrom the antenna elements of M-MIMO antenna array 312 to the UEsincluded in the downlink channel. For example, assume that a first datastream intended for a first UE is transmitted from antenna element 312.0and that a second data stream intended for a second UE is transmittedfrom antenna element 312.1 of M-MIMO antenna array 312. Assume furtherthat a spatial null is to be created at a third UE of a neighboring basestation. By diagonalizing the concatenated downlink channel (formed asdescribed above), the first data stream can be effectively transmittedon a downlink path to the first UE that is spatially orthogonal to thedownlink path on which the second data stream is transmitted to thesecond UE and to the downlink path from base station 300 to the third UE(on which no data stream is transmitted). The same also applies for thesecond data stream, which is effectively transmitted on a downlink pathto the second UE that is spatially orthogonal to the downlink path onwhich the first data stream is transmitted to the first UE and to thedownlink path from base station 300 to the third UE (on which no datastream is transmitted). As such, the first UE would receive only itsintended first data stream and the second UE would receive only itsintended second data stream. Further, the third UE would experience noreceive power due to the transmission.

Returning to FIG. 3, in another embodiment, processor 302 can be furtherconfigured to determine the estimate of the channel from base station300 to the UE served by the neighboring base station using explicitfeedback of the estimate from the UE, as further illustrated in exampleprocess 600 of FIG. 6. Example process 600 is provided for the purposeof illustration only and is not limiting of embodiments. Example process600 is described below with reference to example base station 300 but isnot limited by this description as would be understood by a person ofskill in the art based on the teachings herein.

As shown in FIG. 6, process 600 begins in step 602, which includestransmitting a pilot signal to a UE served by a neighboring basestation. In an embodiment, step 602 is performed by processor 302 and/orother components of base station 300, including M-MIMO antenna array312. In an embodiment, to perform step 602, the UE attaches itself tobase station 300 as a secondary base station. This allows base station300 to instruct the UE to receive the pilot signal on specific time andfrequency resources. The UE can estimate the downlink channel from thepilot signal.

Subsequently, in step 604, process 600 includes receiving from the UE anestimate of a downlink channel to the UE. In an embodiment, step 604 isperformed by processor 302 and/or other components of base station 300,including M-MIMO antenna array 312. In another embodiment, step 604further includes signaling the UE to transmit the estimate of thedownlink channel on specific time and frequency resources. The time andfrequency resources may belong to an uplink control channel of basestation 300.

Process 600 terminates in step 606, which includes determining based onthe estimate of the channel to the UE a transmit precoder matrix totransmit a plurality of data streams, wherein the transmit precodermatrix is configured to produce a transmit beam pattern using M-MIMOantenna array 312 having a spatial null in the direction of the UE. Step606 is similar to step 506 described above in example process 500.

Returning to FIG. 3, in a further embodiment, processor 302 can befurther configured to determine the estimate of the channel from basestation 300 to the UE served by the neighboring base station based onhistorical channel estimates that approximate the estimate of thechannel to the UE, as further illustrated in example process 700 of FIG.7. Example process 700 is provided for the purpose of illustration onlyand is not limiting of embodiments. Example process 700 is describedbelow with reference to example base station 300 but is not limited bythis description as would be understood by a person of skill in the artbased on the teachings herein.

As shown in FIG. 7, process 700 begins in step 702, which includesdetermining an approximate location of a UE served by a neighboring basestation. In an embodiment, step 702 is performed by processor 302 ofbase station 300. In an embodiment, the approximate location of the UEis obtained via a backhaul link from the neighboring base station. Inanother embodiment, processor 302 can use M-MIMO antenna array 312 todetermine the approximate location of the UE. For example, processor 302can use various antenna elements of M-MIMO antenna array 312 todetermine a direction of arrival and an angle of arrival of signalstransmitted by the UE. Processor 302 can also use the receive powers ofsignals transmitted by the UE to approximate the location of the UE.Specifically, the distance to the UE can be calculated based on thereceive powers, and when combined with the angle of arrival, thelocation of the UE can be estimated.

Subsequently, in step 704, process 700 includes retrieving a historicalchannel estimate from a memory based on the approximate location of theUE. In an embodiment, step 704 is performed by processor 302 of basestation 300. In an embodiment, processor 302 retrieves the historicalchannel estimate from memory 314 based on historical channel estimatesstored in memory 314. The historical channel estimates as describedabove may characterize the downlink channel to various locations withina coverage area of a neighboring base station and optionally at varioustimes. In an embodiment, based on the approximate location of the UE(and optionally current time), processor 302 can interpolate orextrapolate stored historical channel estimates to determine ahistorical channel estimate for the UE.

Process 700 terminates in step 706, which includes determining based onthe historical channel estimate a transmit precoder matrix to transmit aplurality of data streams, wherein the transmit precoder matrix isconfigured to produce a transmit beam pattern using M-MIMO antenna array312 having a spatial null in the direction of the UE. Step 706 issimilar to step 506 described above in example process 500, with thehistorical channel estimate used for the estimate of the channel to theUE.

Returning to FIG. 3, in another aspect, as described above, base station300 can use M-MIMO antenna array 312 to create spatial nulls in thereceive direction from a UE served by a neighboring base station or fromthe neighboring base station itself. FIG. 8 illustrates an exampleprocess 800 for creating a receive beam pattern with a spatial null inthe direction of a UE served by a neighboring base station. Exampleprocess 800 is provided for the purpose of illustration only and is notlimiting of embodiments. Example process 800 is described below withreference to example base station 300 but is not limited by thisdescription as would be understood by a person of skill in the art basedon the teachings herein.

As shown in FIG. 8, process 800 begins in step 802, which includesreceiving uplink pilot transmissions transmitted by a UE to aneighboring base station. Step 802 is similar to step 502 described inexample process 500 described above. Subsequently, in step 804, process800 includes determining an estimate of channel from the UE to basestation 300 based on the received uplink pilot transmissions.

Process 800 terminates in step 806, which includes determining based onthe estimate of the channel a receive decoding matrix to receive aplurality of data streams, where the receive decoding matrix isconfigured to produce a receive beam pattern using M-MIMO antenna array312 having a spatial null in the direction of the UE. As discussedabove, the plurality of data streams represent streams being transmittedto the base station by UEs served by the base station. The spatial nullin the direction of the UE of the neighboring base station ensures thattransmissions by the UE are nulled (not received or received withminimal power) at the base station.

Embodiments have been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of embodiments of the present disclosure shouldnot be limited by any of the above-described exemplary embodiments asother embodiments will be apparent to a person of skill in the art basedon the teachings herein.

What is claimed is:
 1. A base station, comprising: a Massive MultipleInput Multiple Output (M-MIMO) antenna array comprising a plurality ofantenna elements; and a processor configured to: determine an estimateof a channel from the base station to a user equipment served by aneighboring base station; determine, based at least in part on theestimate, a transmit precoder matrix; apply the transmit precoder matrixto a plurality of data streams to generate a plurality of pre-coded datastreams; and couple the plurality of pre-coded data streams to a subsetof the plurality of antenna elements of the M-MIMO antenna array totransmit the plurality of pre-coded data streams, wherein the processoris further configured to determine at least one of the transmit precodermatrix or the subset of the plurality of antenna elements of the M-MIMOantenna array to spatially align interference due to transmission of theplurality of pre-coded data streams in one or more spatial directions atthe user equipment.
 2. The base station of claim 1, wherein theprocessor is further configured to receive, using the M-MIMO antennaarray, uplink pilot transmissions by the user equipment to theneighboring base station and calculate the estimate of the channel basedon the received uplink pilot transmissions.
 3. The base station of claim1, wherein the processor is further configured to: transmit, using theM-M MO antenna array, a pilot signal to the user equipment; and receive,using the M-MIMO antenna array, the estimate of the channel from thebase station to the user equipment served by the neighboring basestation.
 4. The base station of claim 1, wherein the processor isfurther configured to: determine an approximate location of the userequipment; retrieve from a memory a historical channel estimate based onthe approximate location of the user equipment; and determine theestimate of the channel based on the historical channel estimate.
 5. Thebase station of claim 4, wherein the processor is further configured touse the M-MIMO antenna array to determine the approximate location ofthe user equipment.
 6. The base station of claim 1, wherein theprocessor is further configured to determine at least one of thetransmit precoder matrix or the subset of the plurality of antennaelements of the M-MIMO antenna array to spatially align the interferencedue to transmission of the plurality of pre-coded data streams in asingle spatial direction at the user equipment.
 7. The base station ofclaim 1, wherein a size of the one or more spatial directions is lessthan a number of the plurality of pre-coded data streams.
 8. A methodperformed by a base station having a Massive Multiple Input MultipleOutput (M-MIMO) antenna array comprising a plurality of antennaelements, comprising: determining an estimate of a channel from the basestation to a user equipment served by a neighboring base station;determining, based at least in part on the estimate, a transmit precodermatrix; applying the transmit precoder matrix to a plurality of datastreams to generate a plurality of pre-coded data streams; andtransmitting the plurality of pre-coded data streams using a subset ofthe plurality of antenna elements of the M-MIMO antenna array, whereinat least one of the transmit precoder matrix or the subset of theplurality of antenna elements of the M-MIMO antenna array is selected tospatially align interference due to transmission of the plurality ofpre-coded data streams in one or more spatial directions at the userequipment.
 9. The method of claim 8, further comprising: receiving,using the M-MIMO antenna array, uplink pilot transmissions by the userequipment to the neighboring base station; and calculating the estimateof the channel based on the received uplink pilot transmissions.
 10. Themethod of claim 8, further comprising: transmitting, using the M-MIMOantenna array, a pilot signal to the user equipment; and receiving,using the M-MIMO antenna array, the estimate of the channel from theuser equipment.
 11. The method of claim 8, further comprising:determining an approximate location of the user equipment; retrievingfrom a memory a historical channel estimate based on the approximatelocation of the user equipment; and determining the estimate of thechannel based on the historical channel estimate.
 12. The method ofclaim 11, further comprising: using the M-MIMO antenna array todetermine the approximate location of the user equipment.
 13. The methodof claim 11, wherein at least one of the transmit precoder matrix or thesubset of the plurality of antenna elements of the M-MIMO antenna arrayis selected to spatially align the interference due to transmission ofthe plurality of pre-coded data streams in a single spatial direction atthe user equipment.
 14. The method of claim 11, wherein a size of theone or more spatial directions is less than a number of the plurality ofpre-coded data streams.
 15. The base station of claim 1, wherein: theplurality of data streams is a first plurality of data streams; thesubset of the plurality of antenna elements of the M-MIMO antenna arrayis a first subset of the plurality of antenna elements of the M-MIMOantenna array; and the processor is further configured to: determine,based at least in part on the estimate, a receive decoding matrix; andapply the receive decoding matrix to a second plurality of data streams,received using a second subset of the plurality of antenna elements ofthe M-MIMO antenna array, to generate a plurality of separated datastreams; and determine at least one of the receive decoding matrix orthe second subset of the plurality of antenna elements of the M-MIMOantenna array to produce a receive beam pattern having a spatial null ina direction of the user equipment.
 16. The base station of claim 15,wherein the processor is further configured to determine at least one ofthe receive decoding matrix or the second subset of the plurality ofantenna elements of the M-MIMO antenna array such that the receive beampattern further has a spatial null in a direction of the neighboringbase station.
 17. The base station of claim 15, wherein the processor isfurther configured to receive, using the M-MIMO antenna array, uplinkpilot transmissions by the user equipment to the neighboring basestation and calculate the estimate of the channel from the base stationto the user equipment based on the received uplink pilot transmissions.18. The base station of claim 17, wherein the processor is furtherconfigured to determine the receive decoding matrix for the M-MIMOantenna array based on the received uplink pilot transmissions.
 19. Thebase station of claim 17, wherein the processor is further configured todetermine an estimate of a channel from the user equipment to the basestation based on the uplink pilot transmissions and to determine thereceive decoding matrix based at least in part on the estimate of thechannel from the user equipment to the base station.
 20. The basestation of claim 15, wherein the receive decoding matrix is configuredto substantially null transmissions from the user equipment at the basestation.
 21. The base station of claim 15, wherein the second subset ofthe plurality of antenna elements of the M-MIMO antenna array overlapsat least in part with the first subset of the plurality of antennaelements of the M-MIMO antenna array.